Baking Sound to F-Curve with Python on Pre-Existing Objects
What I'm struggling with is getting an object that exists already...


You get the objects from bpy.data.objects:

obj = bpy.data.objects[obj_name] #where obj_name is the key / object's name 

#in this case
for i in range(count):
    obj = bpy.data.objects['Fence.%04d' % i] #formatted string 

    #set the keyframe
    obj.keyframe_insert(data_path="scale", frame=1)


Some links:


Add keyframe to a scene property
Python tutorial and documentation
String Format Syntax
Blender Python API documenation